This invention relates to a field-effect transistor (FET) and, more particularly, to a metal-oxide semiconductor FET (MOSFET).
Nowadays a MOSFET is widely used and is required to have more highly integrated construction. A MOSFET, in general, comprises a semiconductor substrate, a source electrode and a drain electrode formed therein, which have a conductivity opposite to that of the substrate, a gate electrode, and a gate insulation layer formed between the substrate and the gate electrode. Hereinafter such a structure is referred to as a "typical structure". There is another structure providing a buried channel layer formed between the source and the drain electrodes having an opposite conductivity to that of the substrate. Hereinafter this structure is referred to as a "buried channel structure".
The most common material used for a gate electrode of a conventional MOSFET is polysilicon. Usually, phosphor or boron is diffused, at a high density, into the polysilicon so as to form a gate electrode made of N.sup.+ polysilicon or P.sup.+ polysilicon in order to lower the resistance of the gate electrode and make stable the work function thereof. Metals having a high melting point such as tungsten, molybdenum or silicide thereof are also well known materials for use as gate electrodes.
However, there are several drawbacks in the conventional MOSFET's using above-described materials. The drawbacks in a MOSFET using N.sup.+ polysilicon for a gate electrode will firstly be described. Since it is desired to set a threshold voltage at about 0.8V in an N-channel FET and at about -0.8V in a P-channel FET generally, the former adopts the typical structure and the latter adopts the buried channel structure. However, in the former, that is, in an N-channel FET, there is a defect of decreasing carrier mobility when circuit integration is conducted. The circuit integration makes an electric field in a channel increase and causes carriers to tend to move through a surface region of a substrate. Thus carrier mobility is decreased, so that operation speed and drive capability of the device are reduced. Further, many hot carriers are produced in the surface region of the substrate and these hot carriers are trapped in an insulation layer. The trapped hot carriers cause a change in electrical characteristics of the device. This change results in a decrease in the reliability of the device.
On the other hand, while a P-channel FET is less subject to the above-described defect because of its buried channel structure, it has the defect of decreasing a threshold voltage when circuit integration is made, because its channel length becomes short with integration (hereinafter this effect is referred to as "short channel effect").
Second, the drawbacks in a MOSFET using P.sup.+ polysilicon for a gate electrode are described. When P.sup.+ polysilicon is used for the gate electrode, buried channel structure is adopted to an N-channel FET and typical structure is adopted to a P-channel FET in order to set a threshold voltage at a designed value. Therefore, as described above, the defect caused by the short channel effect arises in the former and the defect of low carrier mobility and low reliability arises in the latter. In MOSFET's where metals having a high melting point such as tungsten, molybdenum or silicide thereof are used as the gate electrode, the following drawbacks still exist. In these MOSFET's, the density of inpurities must be decreased in order to develop drive capability. Therefore, typical structure must be adopted to both N-channel FET's and P-channel FET's and it was found that drive capability and device reliability are lowered when circuit integration is made.